1. Field of the Invention
The invention relates to a thermal sublimation printing technique, and more particularly, to a thermal sublimation card printer and associated method for printing images onto a card.
2. Description of the Prior Art
Thermal sublimation (or thermal transfer) printers have become increasingly popular due to their excellent full tone printing performance. A thermal sublimation printer drives uses a thermal print head (TPH) to heat a ribbon containing dyes for transferring the dyes onto an object to be printed. In this way, continuous-tone can be formed on the object according to the heating time or the heating temperature.
A thermal sublimation card printer capable of printing images onto all kinds of cards such as business cards, entrance cards or membership cards is developed to satisfy market requirements. A conventional thermal sublimation card printer primarily utilizes a transmission such as a roller to move a card to the thermal print head's position and then utilizes the thermal print head to transfer the dyes from the ribbon onto the card. After transferring the dyes of one color, the transmission mechanism will move the card back to the original position so as to perform a next dye transferring operation of another color.
However, the card's position often deviates from the desired position due to hardware assembly inaccuracy. This therefore results in the existence of an unwanted blank area or the thermal print head of the thermal sublimation card printer performing the printing operation in an area outside the card. The former may decrease the output image quality and the latter may cause the ribbon to break. For example, if the card's position is far behind the ideal position, the thermal print head will start printing an area before a front edge of the card, whereas if the card's position is far ahead of the desired position, the thermal print head will print an area after the rear edge of the card.
Because the card is thicker than conventional paper, if the thermal print head starts to print before the front edge of the card or keeps printing after the rear edge of the card, the ribbon could break when the thermal print head is just about to contact with the card or depart from the edge of the card since the friction force between the card and the thermal print head changes rapidly.
In addition to the above-mentioned hardware assembly inaccuracy, the color contents of the output image also affect the position where the thermal print head finishes printing. For example, the deeper the color of the output image, the greater the tension force when the ribbon departs from the card. This causes the ribbon to push the card forward, resulting in the card's moving speed or distance exceeding a predetermined value. In this way, there is a higher possibility that the ribbon breaks since the thermal print head may keep printing after the rear edge of the card. The above-mentioned problem occurs more frequently when a full size printing is performed.